i have an 04 ext cab 5.3 and when its warm out i avg around 15 city 18 hwy. when its really cold out its more like 12 and 15. Sometimes i can do better on the high way depending on how i drive.
for better gas milage you can make sure your tires are inflated properly or put a little extra in them and make sure your airfilter is clean. If you want to start spending money you can get an exhaust and or a custom tune